With the growth of the wireless communications industry, wireless communications protocols become more sophisticated and demanding in their requirements for complex modulation schemes and narrow channel bandwidths. Transmitter output spectrum requirements become more restrictive and less tolerant of spurious transmissions. Third-generation (3G) and later mobile phone standards may be particularly restrictive.
Switching power supplies are sometimes used to provide power to RF power amplifiers (PAs), since the efficiencies of switching power supplies can be fairly high. Additionally, the magnitudes of switching power supply outputs can be varied to maintain high efficiencies at different output power levels, or to amplitude modulate PAs. However, one drawback to using a switching power supply to power a PA is that a switching power supply uses switching signals at a switching frequency and may generate a ripple signal at the switching frequency and at harmonics of the switching frequency. The ripple signal may be coupled into the PA and appear at the PA's RF output, and may violate output spectrum requirements. Frequency dithering the switching frequency is a technique that is used to spread the frequency content of the ripple signal to meet output spectrum requirements; however, frequency dithering involves changing the switching frequency by using a lower dithering frequency, which may introduce its own dithering frequency ripple signal. Thus, there is a need to reduce the impact of switching power supply ripple on the output spectrum of a PA by dithering the switching frequency of the switching power supply without introducing dithering frequency ripple.